A shearwall is a common structural component of buildings, especially wood frame buildings, that is specifically designed to resist lateral forces due to wind and seismic loads. Typically, shearwalls are constructed on site using plywood or oriented strand board (OSB) sheathing nailed to dimensional lumber studs (e.g., “two by” boards) and plates, together with special hardware connecting the shearwall to the foundation to resist uplift forces.
For conventionally framed wood shearwalls, the structural behavior is well documented and understood. In fact, recent data obtained from the research and testing of such shearwalls has been incorporated into the latest building codes. Current testing procedures are based on protocols requiring that shearwalls be evaluated under cyclic load conditions. During this testing, shear strength and stiffness are determined by subjecting a wall assembly to full-reversal cyclic racking shear loads. The methodology entails anchoring the bottom edge of the wall assembly to a rigid base and applying a force or displacement parallel to the top of the wall. As the wall assembly is racked to specified displacement increments in the plane of the wall, the magnitude of the applied shear force is continuously measured.
The typical failure mode of conventionally framed shearwalls subjected to cyclic loading is characterized as fatigue failure of the nails or other fasteners at the bottom corners of the wall assembly. Thus, the connection between the sheathing and the framing members at this critical location is compromised, resulting in a significant loss of shearwall strength and stiffness. From these observations, shear stresses are concentrated at the bottom corners of the shearwall, so that the connections in these areas are critical to the performance of the shearwall.
Another problem that exists with conventional rectangular shearwalls is their incorporation of commercially available hold-down hardware that is bolted, screwed, or nailed to the bottom portion of the vertical perimeter members. This hold-down hardware is designed to resist the large uplift forces generated at the ends of a shearwall. Such hardware is connected to the foundation with anchor bolts that are necessarily offset to some extent from the vertical members that are being secured. Normally, the hold-down hardware is mounted to the vertical members, and directionally toward the inside of the end post. Due to the eccentricity or offset of the hold-down hardware relative to the centerline of the vertical members, bending moments are created. These bending moments cause increased stresses in the joints between the vertical and lower horizontal member (or base), thereby reducing the capacity of a conventionally framed shearwall.
In resisting various types of stresses encountered in their normal use, shearwalls must exhibit sufficient ductility, a property related to their inherent ability to dissipate seismic energy. For wood framed shearwall structures, building codes allow for a reduction in seismic loads, recognizing that wood shearwalls dissipate seismic energy. A recent development related particularly to the wood frame construction industry is the introduction of prefabricated shearwalls, which are assembled in a manufacturing plant and shipped to construction sites.
One advantage of prefabrication is the ability to incorporate features that strengthen the shearwall assembly, resulting in significantly higher lateral load carrying capacity compared to site built, or conventionally framed, shearwalls of similar dimensions. However, there remains a present need in the art to optimize prefabricated shearwalls in terms of their strength and ductility characteristics.